1. Field of the Invention
The present invention relates to gas detectors; and more particularly, to an improved ionization chamber within the gas detection apparatus and corresponding method.
2. The Prior Art
Chemical agent gas detectors are commonly used to sense the presence of various types of toxic gases for activating audio and visual alarms before the arrival of injurious quantities of such gases.
Historically, diffusion chamber gas detection utilizing ion molecule reactions and subsequent detection of the resulting analyte ions by their differing ion mobilities have been accomplished in the following manner. An air sample of constant flow is presented to an irradiation chamber wherein radioactive particles released from a radiation source contained within the chamber initiate the formation by subsequent molecular reaction of reactant ions of the form (H.sub.2 O).sub.n H.sup.+ and (H.sub.2 O).sub.m O.sub.2.sup.-. These reactant ions generally have values for n and m such that their reduced mobilities are greater than 2.2 cm.sup.2 v.sup.-1 s.sup.-1. These reactant ions pass from the radiation chamber into a diffusion chamber in which they are acted upon by a complex flow path established by labyrinth geometry and velocity control orifices. These diffusion chamber geometries are configured to generally prevent ions with mobilities of greater than 2.0 cm.sup.2 v.sup.-1 s.sup.-1 from passing through the chamber by allowing them sufficient resident time exposure within the diffusion chamber to become neutralized either by collision with the large surface area of the tube walls or by recombination with flow stream entrained ions of opposing polarity.
In the case where the air sample presented to the radiation chamber contains a contaminant gas or gas of interest G, a second molecular reaction between the contaminant gas G, and the reactant ions formed in the irradiation chamber occurs. These newly formed molecules are product ions and within their family resides a group of ions of interest which are of the general form (G).sub.p (H.sub.2 O).sub.r H.sup.+ and (G).sub.s (H.sub.2 O).sub.t O.sub.2.sup.- and which generally have values of p, r, s and t such that their ion mobilities are less than 1.8 cm.sup.2 v.sup.-1 s.sup.-1. When presented to a diffusion chamber sized for preventing passage of molecules of ion mobilities greater than 2.0 cm.sup.2 v.sup.-1 s.sup.-1, these low mobility ions remain in the general flow stream thus avoiding neutralization by collision with the diffusion chamber walls. As a result, these group of interest product ions complete passage through the diffusion chamber with charge retention.
Upon exiting the diffusion chamber the flow stream enters a cavity or collection cup. In this cavity the flow velocity is reduced such that neutralization of the product ions of interest is accomplished by collision with the cavity walls and subsequent charge transfer. When a low electrical potential is connected to the cavity or collection cup surface, a small electric current is generated to replace the cup surface electrons lost in the product ion charge transfer process. With suitable electronic amplification and measurement circuitry, this low level electron transfer current can be sensed and, by monitoring its level, the presence of the product ions of interest and thus the contaminant gas or gas of interest G can be detected.
Current devices which utilize this methodology employ either bipolar or monopolar radiation chambers which require high energy and therefore potentially hazardous radiation sources.
In the case of bipolar chambers, this high energy need arises from diffusion and collection chamber flow and velocity requirements that dictate irradiation chamber flow geometries which permit a high percentage of reactant ion recombination losses to occur before the secondary product ion molecular reactions can take place. The resulting loss of sensitivity is currently overcome by increasing the radiation source strength to a level where the initially formed number of reactant ion pairs is sufficiently large enough to allow a detectable number of product ions to be formed prior to completion of the recombination process.
Radioactive particle and subsequent molecular reaction air stream processing within the monopolar irradiation chamber is accomplished by limiting the free travel path of the radiation source emitted radioactive particles. This is accomplished by coating the radiation source with a thin energy absorbing barrier which consumes in the region of 90% of the radioactive emitted particle energy prior to introduction of these particulates into the air stream. Having undergone this energy reduction processing the emitted particulate velocities are reduced to the point where on average they undergo only the final two travel path collisions. At this low collision velocity the radioactive particle which is generally in the form H.sup.+ collects and retains the collision freed electrons. This effect leaves a neutralized hydrogen particle and two positively charged nitrogen particles within the ionization chamber for each radioactive particle which permeates the barrier. The collision generated nitrogen particles produce a charge enriched zone but only for a subsequent positive reactant and product ionic reactions and only at the expense of incorporating a radiation source of sufficiently high strength to function in the barrier control mode.
Currently, Americium 241 is used as the high strength radioactive source. However, the use of Americium 241 creates a high potential for radiation hazard to the user of the conventional device.
Although a high strength radioactive source is currently in use, current devices are insensistive to relatively minor quantities of chemical agents in the air.